What is the difference between IK and FK in animation on blender
With inverse kinematics (IK) the hands and feet will keep their position when the body is moved. When you switch from FK to IK you need to enable the layers for IK also.

FK and IK have different control bones (custom shapes) for both modes. With IK you control the pose with the large plate shaped widgets at the hands and feet.
The lines you see are pole targets and allow to control the position of the knee or elbow.
IK example

With forward kinematics (FK) each bone from shoulder to wrist needs to be adjusted. This is exactly how bones can be moved right after creation.
FK example

Movement with arms still set to IK but leg to FK
Rigs like Makehuman or MHX allow you to switch from IK to FK separately for the legs and arms. You can even control the influence of the FK/IK controls.

The snap buttons allow copying from the IK position to FK and vice versa.

Here Rigify

MHX